This invention relates to an improved method of operating a blast furnace, and more particularly, to a method of operating the blast furnace in which a part of the usual iron ore feed to the furnace is replaced by prereduced iron ore having a relatively low metallization and a relatively high carbon content. Through the use of prereduced iron ore, both a decrease in the coke requirement and an increase in the overall productivity of the blast furnace is achieved. In the following description, the process is illustratively described as applied to the use of a charge of a prereduced iron ore which is sponge iron. However, as the description proceeds, it will be evident to those skilled in the art that the invention is also applicable to a process that uses prereduced iron ores other than sponge iron obtained from the direct reduction of iron ore.
In general, the production of pig iron in a blast furnace involves charging iron bearing material (iron ore, sinter, pellets, iron or steel scrap, etc.), carbonaceous material as fuel (coke), and flux (limestone or dolomite) into the top of the furnace. A blast of heated air is blown through tuyeres mounted in the bosh into the upper portion of the furnace hearth. A portion of the fuel is burned by the blast air to produce heat for the necessary chemical reactions involved and also for melting the iron. The balance of the fuel and a portion of the gas of combustion is utilized to reduce the iron ore descending through the blast furnace. Typically, in the upper portion of the blast furnace, the unreduced iron ore is partially reduced from FE.sub.2 O.sub.3 (hematite) to FeO (wustite) by the upwardly flowing hot gaseous products from the combustion zone located in the lower portion of the blast furnace. The amount of coke required to supply heat to the blast furnace and to effectuate reduction of the unreduced iron ore is a direct function of the amount and composition of the feed charged to the blast furnace and the desired pig iron production.
In previously proposed processes, the productivity of blast furnaces has been increased through a modification of the burden charged to the blast furnace. The use of prereduced iron ore as part of the charge to a blast furnace has been generally disclosed. However, substantially all of the previously proposed processes charged a highly metallized prereduced iron ore into the blast furnace. It was believed that if the metallization and therefore the metallic iron content of the charge is increased to the highest value possible, the amount of reduction required in the blast furnace could be correspondingly decreased. Therefore, there would be an increase in the productivity of the blast furnace and a decrease in the coke consumption since less coke would be needed to reduce the already partially prereduced iron ore in the charge.
None of the improvements previously suggested have adequately addressed the important overall energy consumption and process efficiency considerations. The need for a higher metallization of prereduced iron ore must be balanced against the greater difficulty and expense of obtaining highly metallized sponge iron as compared to sponge iron with a lower metallization. It has been found that the effect of charginga blast furnace with sponge iron of low metallization and high carburization on the economy and efficiency of the overall blast furnace operation has not been adequately considered.
A need exists for an improved blast furnace operation which will both significantly increase the production of pig iron and decrease the coke consumption while simultaneously maximizing overall economy and efficiency in the production of the prereduced iron ore used as part of the charge to the blast furnace.